If your atrial myxoma causes a distortion of the valve opening (orifice) that could cause blood to backflow into the upper chamber when the lower chamber pumps. This condition will cause less blood pumped into circulation with each heartbeat and the result can be your symptoms.
I have moderate to severe mitral valve regurgitation that was caused by an enlarged left ventricle that distorted the valve opening...it wouldn'rt be unlike a tumor distorting the valve opening. I don't have the symptoms you express, and frankly it is difficult for me to associate your symptoms to a valve disorder...but other conditions concomitant with a valve problem could be the cause for your symptoms?!.
Other causes:
Drop in blood pressure. Inadequate output of blood from the heart. Conditions such as any of the various diseases of the heart muscle (cardiomyopathy), an abnormal heart rhythm (arrhythmia) or a decrease in blood volume may cause inadequate blood flow from your heart.
And many other non-heart related conditions. It seems to me the doctor would dx the symptoms prior to an operation as your symptoms could be serious.
Thanks for sharing and if you have any further questions or comments you are invited to respond. Take care,
Ken
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